This invention relates generally to refrigerant fluids and, more particularly, to the control and prevention of crystallization of refrigerant fluids such as refrigerant fluids composed of a lithium halide, e.g., lithium bromide or lithium chloride, and water.
Some 50 percent of the world's industrial refrigeration equipment is based on the use of absorption refrigeration principles. In the most common application of these principles, heat is used to drive water from the solution in one place (e.g., outside) after which the separated water is first condensed and then evaporated, with associated cooling, and reabsorbed in the salt solution in another place (e.g., inside).
As will be appreciated, the efficiency of an absorption refrigeration process is typically dependent on a number of factors. The boiling point of the absorption fluid is often an important factor because it reflects not only the heat input needed to separate the water from the solution but also, through its connection to the thermodynamic activity of the water in the solution, the drive to reabsorb it in the cooling part of the cycle. For high efficiency, a high boiling point is generally considered desirable.
Unfortunately, in normal practice, limits may be imposed on the boiling point of the refrigerant fluid because of intervention related to crystallization of salt hydrates in lower temperature parts of the equipment if the concentration of salt becomes too high. More specifically, lithium bromide and water compositions having suitably high boiling points are typically prone to crystallization of LiBr hydrates, the formation of which can undesirably block or otherwise obstruct a system piping. In particular, the inadvertent crystallization, such as in block form, of the lithium bromide dihydrate can be catastrophic. Consequently, lithium bromide and water refrigerant fluids have commonly been restricted to compositions which contain no more than about 25 mol % (61.6 wt %) LiBr and have a boiling point of only about 160.degree. C.
One approach to avoiding such crystallization has involved the addition of trace additives such as may serve to deactivate the sources of heterogeneous nucleation and may also favorably affect the probability of homogeneous nucleation. In theory, such an approach would only necessitate the presence of such additives in relatively small quantities. Thus, the thermodynamic and mass transport properties of the solutions, which have generally been thoroughly characterized by the industry, desirably would only be minimally perturbed by such addition.
Unfortunately, while experience has shown that such a technique may be effective when used in conjunction with smaller sized samples, experience has also shown that process thermodynamics are not as readily or easily overcome when such a technique is applied on industrial facility scale quantity solution samples.
Another approach, much pursued by industry, involves reducing or lowering the activity of the LiBr by the addition of a Lewis acid, such as ZnCl.sub.2 or ZnBr.sub.2, in order to yield high boiling solutions having low melting temperatures. The acid-base interaction associated with such an approach produces low basicity anions, like ZnCl.sub.4.sup.2-, ZnBr.sub.4.sup.2- and their mixed ligand variants. To be effective, a rather large mole fraction of tetrahalozincate anion is generally required. As a result, such an approach may undesirably decrease the water content of the solution significantly and undesirably alter other properties of the solution, such as the density and viscosity.
Still another approach is to use an additive which, rather than chemically interacting with the solution, generally takes advantage of the ideal mixing laws for components of binary solutions in which each of the binary solution components are insoluble in the crystal lattice of the other solution component. Thus, mixtures of LiBr.2H.sub.2 O and LiSCN.2H.sub.2 O can be used to obtain a pseudo-binary eutectic mixture which has a lower melting point than either of the components taken alone. On dilution with additional water, the crystallization of the dihydrate, at ambient temperature, is suppressed. This admixture perturbs the properties of the LiBr+H.sub.2 O system rather minimally because SCN.sup.-, acts as a "pseudo-halogen," resulting in solution properties similar to those of the bromide. In particular, the boiling point is relatively unchanged from that of the LiBr solution of the same water content. Unfortunately, LiSCN is generally much more expensive than LiBr and relatively large amounts of the LiSCN are generally required for this approach to be successful.
Thus, there is a need and a demand for a method of avoiding or suppressing unwanted crystallization of or from such a refrigerant fluid without undesirably depressing the boiling point of such refrigerant fluid, as well as a need and a demand for an aqueous lithium halide solution refrigerant fluid (such as of lithium bromide) having a relatively high boiling point and which desirably minimizes or avoids crystallization such as may undesirably obstruct or block system piping.